Raymond Robijns wrote:
> why is the middle C (Midi note 60) called C3 in one program and in another
> program C5?

because keyboards/pianos are different large (number of keys) and most
developers numbers its own deepest C key C0 or C1. Depending on the
number of deep octaves available the middle C can have any octave
index.

In standard Midi the middle C is always note 60, it does not support
deeper note than 0 even if a synth could play deeper notes.